An antenna circuit of a mobile communication terminal allows an antenna to transmit or receive predetermined radio signals. In order to optimize the transmit/receive radiation performance of the antenna, impedance matching must be exactly performed in the optimal state.
Therefore, the antenna circuit includes capacitors and inductors, and adjusts values of the capacitors and the inductors, thereby exactly performing impedance matching. In general, the impedance matching of the antenna is achieved in a state that the mobile communication terminal is positioned in a free space.
Meanwhile, the mobile communication terminal may be used by a user in a state that the user handholds the body thereof and adheres a speaker to the ear of the user due to the characteristic of the mobile communication terminal, or may be used by the user through an ear-phone in a state that the body of the mobile communication terminal is put into a pocket or a bag of the user. As the user uses the mobile communication terminal by gripping the mobile communication terminal with the hand of the user and closely touching the speaker thereof to the ear of the user, or as the user uses the mobile communication terminal put into the pocket or the bag, the conditions for the impedance matching of the antenna vary, so that the transmit/receive radiation performance of the antenna subject to impedance matching in the free space may be degraded.
Therefore, the mobile communication terminal employs an impedance matching apparatus having an antenna circuit to automatically adjust the impedance of the antenna when the conditions for the impedance matching of the antenna vary, so that the antenna has the optimal transmit/receive radiation performance.
To this end, the impedance matching apparatus includes a coupler to detect reflected power and forward power subject to coupling in the coupler, and to change the capacitance of a variable capacitor according to the reflected power and the forward power, so that impedance matching can be performed.
However, according to the impedance matching apparatus of the related art, the transmit power is measured by an external detection unit, so that the accuracy in the measurement of the transmit power may be degraded.